Ah, that's not the stock bumper. I suppose you had it painted but not the rest of the car, hence why it looks a diff. color at certain angles. I can't imagine the entire car was painted and the angle makes it do that.

I'm no painter, but everything I've ever heard/seen regarding laying metallic paint down, stated direction is everything. If one piece is painted using a spray motion different from an adjacent piece, then they'll look different. Maybe not at every angle, but at some. Which probably explains the phenomenon here.